There are certain types of industrial operations which produce enormous quantities of pulverulent material. Orderly transfer of such material from the units that produce it to a transportation system presents substantial problems which are aggravated by the nature of the material which makes it inherently difficult to design systems for its orderly handling.
A severe problem is presented by the fly ash produced by the high efficiency coal burning furnaces and collected in the precipitators of electric power generating plants. A large coal burning power facility can produce many tons of fly ash an hour; and it is extremely difficult material to handle because it is about as fine as talcum powder, is very abrasive, and has a great tendency to lump and cake in hoppers and conveyors. This makes it hard to obtain a reasonably even flow of material into a device such, for example, as a pressure-type pneumatic conveyor which is very efficient for moving pulverulent material.
In the system disclosed and claimed in U.S. patent application No. 137,482 filed Apr. 4, 1980, pulverulent material is moved from the receiving hoppers of precipitators into a pressurized air conveyor pipeline by a system of mechanical transfer cnveyors from which the material drops by gravity into a pressure mechanical conveyor; and from the pressure mechanical conveyor the material passes into the pressurized air conveyor pipeline.
The receiving hoppers and the mechanical transfer conveyors operate at about atmospheric pressure; while the pressure mechanical conveyors are cyclically switched between receiving hopper pressure and a pressure equal to that in the pressurized air conveyor pipeline.
In a single feeding cycle the bottom gate valve which is between the pressure mechanical conveyor and the pressurized air conveyor pipeline is originally closed, and the top gate valves between the mechanical transfer conveyors and the pressure mechanical conveyors are open. The transfer mechanisms for moving material from the receiving hoppers, the conveyor means in the pressure mechanical conveyor, and the pipeline operate continuously. During a material receiving phase of the cycle the pressure mechanical conveyors must, of course, be at the same pressure as are the receiving hoppers and the transfer conveyors; and this pressure is about atmospheric.
At the end of the material receiving phase, which is controlled to effectively fill the pressure conveyor casing, the top gates are closed, a vent valve from the pressure mechanical conveyor is closed, and a pressure valve is opened to raise the pressure in the pressure mechanical conveyor casing to the same level as that in the pressurized air conveyor pipeline. When this pressure is equalized, the bottom gate valve is opened and the continuously operating pressure conveyor mechanism moves the material out of the pressure conveyor casing into the pressurized air conveyor pipeline during a material discharge portion of the cycle.
After effectively all the material in the pressure mechanical conveyor casing has been discharged, the bottom gate and the pressure valve are closed and the vent valve for the pressure mechanical conveyor casing is opened to return that unit to receiving hopper pressure, whereupon the top gates are opened to start a new cycle. The cycle is controlled to effectively entirely fill and empty the pressure mechanical conveyor casing.
In a plant the receiving hoppers may be divided into two or more sets, in application No. 137,482 and here for convenience limited to two sets (I and II); and each set has an A section and a B section. While the pressure mechanical conveyor of the I-A section is in the first, or filling portion of a cycle, the pressure mechanical conveyor for the I-B section is in the second, or discharge portion of the cycle. The pressure mechanical conveyors of the II-A and II-B sections are one-quarter cycle out of phase with those of the I-A and I-B sections.
In addition, the two ends of a pressure mechanical conveyor casing receive material simultaneously from two transfer conveyors; and in the casing are conveyors which are driven to move material from both ends toward the transverse median plane of the casing to be fed into the pressurized pipeline through a bottom gate valve which is on that transverse median plane.
Preferably the transfer conveyor mechanisms and the pressure conveyor mechanisms are augers; and in the latter units there are two augers of opposite hand on a single shaft.